An Anionic Aluminabenzene Bearing Aromatic and Ambiphilic

Subscriber access provided by Service des bibliothèques | Université de Sherbrooke

Communication

An Anionic Aluminabenzene bearing Aromatic and Ambiphilic Contributions Taichi Nakamura, Katsunori Suzuki, and Makoto Yamashita J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 16 Jun 2014 Downloaded from http://pubs.acs.org on June 17, 2014

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Journal of the American Chemical Society is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 5

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Journal of the American Chemical Society

An Anionic Aluminabenzene bearing Aromatic and Ambiphilic Contributions Taichi Nakamura, Katsunori Suzuki,* and Makoto Yamashita* Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, 1128551 Tokyo, Japan Supporting Information Placeholder ABSTRACT: The synthesis and structure of an anionic alumi-

nabenzene, which is the first example of an aluminum-containing heterobenzene, are reported. The molecular structure of this aluminabenzene exhibits a planar six-membered ring and the absence of any bond alternation between its unsaturated bonds is consistent with the structural criteria of aromaticity. Theoretical calculations and the NMR spectroscopic analysis of this anionic aluminabenzene furthermore suggest that, in addition to the aromatic conjugation of six π-electrons, an ambiphilic contribution from a Lewis acidic aluminum center and an anionic pentadienyl moiety is present. Due to this contribution, the aluminabenzene is able to react with Lewis bases such as 4-dimethylaminopyridine and electrophiles such as methyl iodide.

Benzene is one of the most fundamental organic molecules. Sixmembered 6π aromatic systems similar to benzene typically consist of carbon and nitrogen atoms. The incorporation of heavier main group elements from the third, fourth, or higher rows of the periodic table into such aromatic systems has recently attracted much attention, mostly because benzene rings containing heavy elements best describe the structural motif of “heavy aromaticity”.1 Whereas the synthesis of a variety of benzene derivatives with elements from group 14 and 15, e.g. as sila-,2 germa-,3 phospha-,4 arsa-,5 stiba-,6,7 and bisma-benzenes7 has been reported and allowed further insight into their aromaticity, reports on the isolation of analogues with group 13 elements still remain limited.8 The incorporation of an electron-deficient group 13 element in these benzene derivatives with 6π electrons results in the presence of an empty sp2 orbital (A, Chart 1), which should lead to considerable instability. It is therefore not surprising that so far, no example of a heterobenzene containing a group 13 element and a vacant sp2 orbital has been reported. However, several examples of corresponding Lewis base-coordinated compounds (B, Chart 1) and anionic complexes (C, Chart 1) have been synthesized and these show remarkable stability, once the vacant sp2 orbital is occupied. Although several boron-containing heterobenzene derivatives are known, only one example containing the heavier group 13 element gallium has been reported in the form of an anionic galabenzene, which was spectroscopically characterized in solution.9 The corresponding aluminum analog, aluminabenzene, still remains unknown despite several theoretical studies suggesting its aromaticity.10 Examples of an aluminum-carbon double bond also remain elusive, even though it represents a fundamental class of chemical bonds. The synthetic difficulty to isolate aluminabenzenes may be attributed to the energetically unfa-

vorable orbital overlapping between the 3p orbital of aluminum and the 2p orbital of carbon to form an unsaturated bond. The closest example to aluminum-containing aromatic compounds with unsaturated bonds between aluminum and carbon is a dianionic aluminacyclopentadiene, which was recently synthesized, characterized, and interpreted as an aluminum analog of the cyclopentadienyl anion, in which the endocyclic Al-C bonds were found to be contracted in comparison to its precursor.11

Chart 1. Heterobenzenes Containing a Group 13 Element

Herein, we report the synthesis and characterization of the anionic aluminabenzene 1, which represents the first isolated example of an aluminum-containing benzene ring (Chart 2). The crystallographic analysis, as well as the theoretical and spectroscopic examinations, suggest for this compound a significant contribution of the ambiphilic resonance form 1B in addition to the aromatic resonance form 1A. This notion was supported by the ability of 1 to sequentially react with a Lewis base and an electrophile.

Chart 2. Mesomeric Resonance Structures of 1

Aluminabenzene 1 was synthesized following the synthetic strategy outlined in Scheme 1. The reaction of bis(triisopropylsilyl)diyne 2 with DIBAL-H, followed by addition of nBu2SnCl2, resulted in the formation of chloro-bridged dinuclear aluminacyclohexadiene 3 in 32% yield (for a discussion of the reaction mechanism, see the supporting information). Subsequent treatment of 3 with pyridine afforded pyridine-coordinated aluminacyclohexadiene 4 in 78% yield, which was used as a precursor for 1. Although attempts to abstract a proton from 4 with bulky bases such as TMPLi in order to generate the corresponding neutral Lewis base-coordinated aluminabenzene (B, Chart 1) were unsuccessful, the reaction of 4 with mesityllithium in Et2O at – 35 °C gave the anionic aluminabenzene 1-Li(Et2O). Unfortunately, the attempted recrystallization of 1-Li(Et2O) failed to produce single crystals suitable for X-ray crystallographic analysis, but the

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 5

optimization of the recrystallization conditions showed that the Et2O solvate molecule coordinating the lithium cation could be exchanged through the addition of other coordinating solvents such as 1,2-dimethoxyethane (DME). Depending on the conditions, recrystallization of 1-Li(Et2O) in the presence of DME deposited two types of crystals, 1-Li(DME) or 1-Li(DME)3, both of which were suitable for X-ray crystallographic analysis. Treatment of 1-Li(Et2O) with DME and a subsequent recrystallization from toluene at –35 °C resulted in the deposition of single crystals of 1-Li(DME) in 72% yield. Recrystallization of 1-Li(DME) from DME at –35 °C afforded single crystals of 1-Li(DME)3 in 57% yield.

Scheme 1. Synthesis of 1 and Recrystallization Conditions

Figure 1. Molecular structures of 1-Li(DME) (A) and 1Li(DME)3 (B) (50% thermal ellipsoid probability). Hydrogen atoms and solvate molecules are omitted for clarity and only selected atoms are labelled.

Reaction conditions: a) DIBAL-H, hexane; b) nBu2SnCl2, hexane; c) pyridine; d) MesLi, Et2O; e) treatment with DME, then recrystallization from toluene at –35 °C; f) recrystallization from DME at –35 °C. The molecular structures of 1-Li(DME) and 1-Li(DME)3 are shown in Figure 1. In 1-Li(DME), an interaction between the lithium cation and the aluminabenzene ring was observed [Al1– Li1 = 2.982(4) Å, C–Li1 = 2.290(4)–2.665(4) Å]. Due to the steric repulsion between the solvated cation [Li(DME)]+ and the mesityl substituent, the aluminum atom is situated slightly above the C1–C5 plane, resulting in an angle of 16.78(11)° between the C1–C2–C3–C4–C5 least square plane and the Al1–C1–C5 plane (see Fig. S7 in the supporting information). Due to the solvation by three molecules of DME, the lithium cation in 1-Li(DME)3 is completely separated from the anionic moiety (Al1-Li > 7 Å) in the solid state. The sum of the internal angles of the sixmembered ring in the aluminabenzene is 720°, indicating a completely planar structure. The Al center in 1-Li(DME)3 exhibits a three-coordinated geometry and the planar structure is supported by the sum of angles (360°) around the Al center. The C-C bond lengths in the aluminabenzene rings are almost equal and in the range of 1.403(3)–1.420(3) Å and 1.403(3)–1.414(3) Å for 1Li(DME) and 1-Li(DME)3, respectively. The observed values are between typical carbon-carbon single and double bonds and comparable to those of benzene.12 The endocyclic Al1-C1 and Al1-C5 distances are 1.930(2) Å/1.928(2) Å for 1-Li(DME) and 1.924(2) Å/1.922(2) Å for 1-Li(DME)3. These are significantly shorter compared to the exocyclic Al1-C6 single bond lengths of 1.984(2) Å for 1-Li(DME), and 2.010(2) Å for 1-Li(DME)3. The observed metric parameters are also comparable to those of the previously reported dianionic aluminacyclopentadiene.11 The combined structural analysis allows the conclusion that aluminabenzene 1 exhibits a planar structure and contains unsaturated bonds without bond alternation, which is in agreement with the structural criteria of aromaticity.

In order to elucidate the electronic structure of anionic aluminabenzene 1, we conducted DFT calculations13 at the B3LYP/631G(d) level, because there, the structural optimization for the real anionic aluminabenzene 1 was able to reproduce the experimentally observed structure of the anionic aluminabenzene moiety in 1-Li(DME)3. The optimized structure is moreover comparable to the parent anionic aluminabenzene [AlC5H6]–, which was previously calculated at the same level of theory and may also have a planar structure with a delocalized π-electron system,10a except for a slight expansion (0.9%) of the Al-C distances in 1 (1.924 Å/1.926 Å) relative to the parent aluminabenzene (1.908 Å). This elongation can mostly likely be attributed to the repulsion between the sterically demanding mesityl and silyl groups. An examination of the frontier orbital situation showed that similar to benzene, six π-MOs are filled with six π-electrons in 1 (Figure 2). But in contrast to the HOMOs and LUMOs of benzene, which consist of two pairs of degenerate orbitals, the HOMO and LUMO of 1 are, owing to its unsymmetric structure, not degenerate. The HOMO–11 of 1 expands mainly over the pentadienyl moiety, suggesting that the conjugation between the 3p atomic orbital of aluminum and the 2p atomic orbital of carbon is less effective in 1 relative to benzene. The bond order analysis based on the Wiberg bond index (WBI)14 suggested an unsaturated character for the aluminabenzene ring of 1, especially for the Al–C bonds. The bond order values for the C-C bonds were estimated to be 1.38– 1.48, which fall between those of single and double bonds. Bond order values of 0.70 and 0.71 were obtained for the endocyclic AlC bonds, which is approximately 1.4 times higher than that of the exocyclic Al–C(Mes) single bond (0.51). The calculated NPA charge15 indicated a charge localization in 1. Although the entire aluminabenzene ring carries a negative charge, the local charge on the Al atom is estimated to be positive (+1.5). In contrast to that, the charges on the five carbon atoms are calculated to be negative (ortho-C = –1.4, meta-C = –0.19, and para-C = –0.35). Previously reported calculations for the parent anionic aluminabenzene [AlC5H6]– revealed a similar trend for the charge separation (Al = +0.60, ortho-C = –0.65, meta-C = –0.08, para-C = –0.14). Considering the NPA charge, the charge-localized character of the aluminabenzene ring in 1 can be interpreted as the result of a neutral Lewis acidic aluminum and an anionic pentadienyl moiety, consistent with resonance structure 1B. The differences in the atomic charges between 1 and [AlC5H6]– could then be feasibly attributed to substituent effects of the electronegative mesityl and the electropositive ortho-silyl groups. In order to further investi-


Page 3 of 5

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


gate the aromatic character of 1, we carried out an analysis of the nuclear independent chemical shifts (NICS)16,17 at the B3LYP/6311+G(d,p) level, which provided NICS(0) and NICS(1) values of –1.53 and –3.75, respectively. Even though these values are smaller than the reported NICS(0) and NICS(1) values for [AlC5H6]– (–4.13/–5.63), which were calculated at the same level,10d they still suggest an aromatic contribution to the structure of 1. The lower NICS values of 1 might originate from a substantial contribution of the charge localized structure 1B, which should have no aromatic ring current. Nevertheless, the contribution from the aromatic resonance structure 1A should not be neglected, as the WBIs suggest an unsaturated bond character for the aluminabenzene ring.

Figure 2. Selected molecular orbitals of 1 calculated at the B3LYP/6-31G(d) level. The LUMO is localized on the mesityl group (see the supporting information). The experimental NMR spectra of 1 are also consistent with the presence of a resonance hybrid with contributions from both aromatic 1A and ambiphilic 1B. In the 1H NMR spectrum of 1Li(DME), the resonances for the meta- and para-protons attached to the aluminabenzene ring were observed at 8.16 (meta) and 5.98 (para) ppm, which is characteristic for aromatic compounds. In accordance with the 1H NMR analysis, the signals for the corresponding para-, ortho-, and meta-carbon atoms in the 13C NMR spectrum were also observed in the aromatic region (104.6, 121.0, and 151.7 ppm, respectively). However, the chemical shift differences (∆δ) for the hydrogen and carbon atoms of the ring are pronounced: ∆δH = 2.18 ppm between para- and meta-hydrogen atoms, as well as ∆δC = 46.1 ppm between para- and meta-carbon atoms. The calculated NPA charges for aluminabenzene 1 suggested that atoms in para-position should carry more negative charge than those in meta-positions. Consequently, the hydrogen and carbon atoms in para-position may be magnetically shielded, resulting in an up-field shift relative to the atoms in meta-position. These chemical shift differences are similar to those observed for

the 1,5-bis(trimethylsilyl)pentadienyl anion,18 corroborating a considerable contribution from canonical form 1B. For lithium cations coordinated by an anionic aromatic ring, the chemical shift of the 7Li NMR can be used to probe the for an evidence of the aromaticity, as the diatropic ring current magnetically shields the lithium cation.17,19,20 According to the structural analysis, the lithium cation in 1-Li(DME) is situated above the aluminabenzene ring, and should therefore be shielded by the aromatic ring current. Indeed, the 7Li resonance of 1-Li(DME) in C6D6 was observed to be up-field shifted at –4.5 ppm, which is comparable to other aromatic lithium salts19,20 and demonstrates the aromaticity of aluminabenzene 1. In contrast, the 7Li resonance for a DME solution of 1-Li(DME), where 1-Li(DME)3 may form in solution, was observed at –1.2 ppm. The down-field shift of this resonance relative to that in C6D6 can be explained by the increased distance between the lithium cation and the aluminabenzene as a result of the exhaustive solvation with DME. The reactivity of aluminabenzene 1 also manifested the contribution of the ambiphilic structure 1B (Scheme 2). Reaction of 1Li(Et2O) with the strong Lewis base 4-dimethylaminopyridine (DMAP) in toluene produced the Lewis acid-base complex 5 cleanly in 91% yield, which reflects the Lewis acidic nature of the Al center. On the other hand, methyl iodide did not react with 1Li(Et2O) in C6D6. However, Lewis complex 5 smoothly reacted with methyl iodide in toluene to give methylated compound 6 in 81% yield. This result may indicate that the nucleophilicity of the conjugated carbanion in 1B is initially weak, but significantly increased in carbanion 5 due to the potentially induced dearomatization, resulting from the coordination of the Lewis base to the Al center. The molecular structure of 5 (Figure 3) was determined by X-ray crystallography and showed a tetrahedral coordination geometry for the Al center, which confirmed the aforementioned dearomatization. The observed C–C bond lengths of the aluminacycle [1.398(4)–1.427(4) Å] are comparable to those in 1Li(DME) and 1-Li(DME)3, but the Al–C bond lengths were found to be substantially increased [1.974(3)/1.987(3) Å]. This bond elongation indicates that the coordination of DMAP hampers the cyclic π-conjugation between the Al atom and the pentadienyl moiety. The NMR analysis of Lewis complex 5 further supported the notion of a dearomatization as a result of the coordination of a Lewis base. Although the 1H NMR resonances for the protons of the aluminacycle in 5 (6.07 and 8.17 ppm) were, probably due to the anionic charge of the pentadienyl moiety, comparable to those of 1, the 7Li NMR resonance for 5 (–1.8 ppm) was observed to be 2.8 ppm down-field shifted relative to that of 1-Li(DME) (–4.5 ppm). The X-ray crystallographic analysis of 5 showed that the lithium cation is situated above the ring. The observed down-field shift of the 7Li NMR signal of 5 relative to 1 is also consistent with a dearomatization resulting from the coordination of DMAP to the Al center.

Scheme 2. Reactions of 1-Li(Et2O) with DMAP and MeI.

Reaction conditions: a) DMAP, toluene; b) MeI, toluene.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 5

vative Areas “Stimulus-Responsible Chemical Species for Creation of Functional Molecules” (No. 24109012) and “New Polymeric Materials Based on Element-Blocks” (No. 25102538) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT). Computational calculations were carried out using the Research Center for Computational Science, Okazaki, Japan. X-ray crystallographic analyses were partially conducted in the Research Hub for advanced Nano Characterization of the University of Tokyo, supported by MEXT, Japan. The authors would like to thank Prof. T. Hiyama, Research and Development Initiative, Chuo University for the provision of an X-ray diffractometer.

REFERENCES

Figure 3. Molecular structure of 5 (50% thermal ellipsoid probability). All hydrogen atoms and solvate molecules are omitted for clarity and only selected atoms are labelled. Based on these results, the structure of aluminabenzene 1 is best described as a resonance hybrid with contributions from both aromatic (1A) and the ambiphilic (1B) resonance structures. The substantial contribution of ambiphilic 1B may arise from the relatively weak aluminum-carbon multiple bond in the aluminabenzene. Sequential reactions of 1 with DMAP and methyl iodide were able to demonstrate that 1 reacts as a Lewis pair, where Lewis acidic and basic centers coexist, which is consistent with resonance structure 1B.

ASSOCIATED CONTENT Supporting Information Experimental details, crystallographic data, DFT calculations and a full citation of reference 13 are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors [email protected] [email protected]

Notes

The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by a Grant-in-Aid for Young Scientists (B) (No. 25810029) from the Japan Society for the Promotion of Science (JSPS) and Grant-in-Aid for Scientific Research on Inno-

(1) (a) Lee, V. Y.; Sekiguchi, A. In Organometallic compounds of lowcoordinate Si, Ge, Sn, and Pb: from phantom species to stable compounds. John Wiley & Sons, Ltd. 2010. (b) Ashe, A. J., III Acc. Chem. Res. 1978, 11, 153. (c) Balaban, A. T. Adv. Heterocycle. Chem. 2010, 99, 61. (d) Lee, V. Y.; Sekiguchi, A. Angew. Chem. Int. Ed. 2007, 46, 6696. (e) Lee, V. Y.; Sekiguchi, A.; Ichinohe, M.; Fukaya, N. J. Organomet. Chem. 2000, 611, 228. (f) Tokitoh, H. Acc. Chem. Res. 2004, 37, 86. (2) Wakita, K.; Tokitoh, N.; Okazaki, R.; Nagase, S. Angew. Chem. Int. Ed. 2000, 39, 634. (3) Nakata, N.; Takeda, N.; Tokitoh, N. J. Am. Chem. Soc. 2002, 124, 6914. (4) Märkl, G. Angew. Chem. Int. Ed. 1966, 5, 846. (5) Ashe, A. J. III J. Am. Chem. Soc. 1971, 93, 3293. (6) Ashe, A. J. III J. Am. Chem. Soc. 1971, 93, 6690. (7) Ashe, A. J. III; Diephouse, T. R.; El-Sheikh, M. Y. J. Am. Chem. Soc. 1982, 104, 5693. (8) (a) Fu, G. C. Adv. Organomet. Chem. 2001, 47, 101. (b) Bosdet, M. J. D.; Piers, W. E. Can. J. Chem. 2008, 86, 8. (9) Ashe, A. J. III; Al-Ahmad, S.; Kampf, J. W. Angew. Chem. Int. Ed. Engl. 1995, 34, 1357. (10) (a) Priyakumar, U. D.; Sastry G. N. J. Org. Chem. 2002, 67, 271. (b) Priyakumar, U. D.; Sastry G. N. Proc. Indian Acad. Sci. 2003, 115, 49. (c) Sieswari, A.; Priyakumar, U. D.; Sastry G. N. THEOCHEM 2003, 663, 145. (d) Ebrahimi, A. A.; Ghiasi, R.; Foroutan-Nejad, C. THEOCHEM 2010, 941, 47. (e) Casanova, D.; Alemany, P.; Alvarez, S. J. Comput. Chem. 2010, 31, 2389. (11) Agou, T.; Wasano, T.; Jin, P.; Nagase, S.; Tokitoh, N. Angew. Chem. Int. Ed. 2013, 52, 10031. (12) Minkin, V. J.; Glukhovtsev, M. N.; Simkin, Y. B. In Aromaticity and Antiaromaticity; Electronic and Structural Aspects; Wiley: New York, 1994. (13) Frisch, M. J. et al. Gaussian09, revision C.01. Gaussian Inc. (14) Sizova, O. V.; Skripnikov, L. V.; Sokolov, A. Y. THEOCHEM 2008, 870, 1. (15) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899. (16) Schleyer, P. v. R.; Maerker, C.; Dransfeld, A.; Jiao, H.; Hommes, N. J. R. v. E. J. Am. Chem. Soc. 1996, 118, 6317. (17) Chen, Z.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.; Schleyer, P. v. R. Chem. Rev. 2005, 105, 3842. (18) Yasuda, H.; Nishi, T.; Lee, K.; Nakamura, A. Organometallics 1983, 2, 21. (19) Paquette, L. A.; Bauer, W.; Sivik, M. R.; Buhl, M.; Feigel, M.; Schleyer, P. v. R. J. Am. Chem. Soc. 1990, 112, 8776. (20) Sekiguchi, A.; Matsuo, T.; Watanabe, H. J. Am. Chem. Soc. 2000, 122, 5652.


Page 5 of 5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

5

An Anionic Aluminabenzene Bearing Aromatic and Ambiphilic

Recommend Documents